Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Nov 11 2022 15:56:55 FOUS30 KWBC 111556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1055 AM EST Fri Nov 11 2022 Day 1 Valid 12Z Fri Nov 11 2022 - 12Z Sat Nov 12 2022 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS MUCH OF THE APPALACHIANS... ....16Z Outlook Update... The ongoing forecast is generally on track. Only minimal changes have been made to the outlook to trim probabilities in the southern Appalachians where the short-term heavy-rain threat has largely ended. The combination of orographic lift, convergence, and modest instability could still promote locally enhanced rain rates in higher elevations of western North Carolina (northward into Virginia later) and the Slight was maintained in those areas for what should be mostly a late-morning through mid-afternoon flash flood risk. A very slight eastward nudge in the Marginal risk area was made for the general I-95 corridor in Maryland/DC this afternoon.=20 Convective bands on the east side of TD Nicole should produce localized 0.5-1.5 inch/hr rain rates as a moist, buoyant airmass migrates northwestward into the region. Given the urbanized nature of ground conditions, a few areas of runoff/ponding could occur should one or two training convective bands materialize across the region today through early tonight. Remaining portions of the Slight/Marginal risk areas are unchanged across the Appalachians. Despite relatively dry antecedent conditions, areas of 1.5 inch rainfall totals in a 3-hour period could approach FFG thresholds especially in West Virginia and western Pennsylvania today. Localized areas of excessive runoff are possible in this regime. The ongoing Marginal risk area in Texas/Louisiana remains unchanged. See the prior discussion below for more information. Cook ....Previous discussion... ....Southern and Central Appalachians, eastern parts of the Ohio Valley, and the Interior Northeast... While the center of Tropical Depression Nicole continues to gradually weaken and lift northward into the southern Appalachians today, an approaching deep upper-level trough over the central U.S. will help surge an extremely moist airmass (above the 95th percentile compared to climatology per the 00z GEFS) across the Appalachians and East Coast. Widespread PWATs between 1.5-2.0" will encompass a very large area as the tropical environment around Nicole escapes farther into the mid latitudes. Strong mid-level moisture flux and a potent 50 kt low-level jet is anticipated to create sufficient upslope across the southern/central Appalachians and Blue Ridge Mountains through this evening to produce maximum rainfall totals up to 8 inches. 00z HREF depicts neighborhood probabilities of 20-25% for 6-hr QPF exceeding 3 inches across the western North Carolina mountains early this evening. These amounts would near and possibly locally exceed the 2 to 4 inch 6-hour FFG throughout the region. Farther north, a right entrance region of a strengthening jet streak aided by the approaching upper-level trough and upper-level outflow from Nicole will produce widespread shower activity over the eastern Ohio/Tennessee Valleys. Rainfall rates should remain below 1 inch per hour, but could saturate soils enough over eastern Ohio and points northeastward to cause minor concerns when the final line of moderate to heavy rainfall moves through. The aforementioned cold front is expected to merge with Nicole's remnants after 00z near the eastern Ohio Valley and could include enough frontogenesis to support rainfall rates near or just over 1 inch per hour. This is highlighted by 00z HREF probabilities of 20-35% for 1-hour QPF exceeding 1 inch between southeast Ohio and western Pennsylvania between 22z Friday and 06z Saturday. Much of this region has been dry as of late, but precluding rainfall could alter initial soil conditions and make more sensitive terrain more vulnerable to rapid runoff. Total areal-average rainfall amounts of 2 to 4 inches are forecast within the Slight Risk area. Overall, the entire system is moving quite quickly and instability is minimal, which limits the magnitude of any major flash flood impacts. For eastern areas of the Mid-Atlantic from eastern North Carolina to New Jersey, better instability (MUCAPE near 500 J/kg per the 00z GFS) will support some potential for higher rainfall rates, but the scattered storms should zip northeastward with the strong southerly flow and produce little flash flood risk. The biggest change to the updated outlook was to shift the western extent of the MRGL and SLGT eastward about 30 miles between Ohio and western New York in order to capture the eastward trend of the heaviest QPF. ....Central Texas to eastern Louisiana... The southern edge of the cold front extending into the southern Plains is likely to spawn convection early this morning ahead of a sharp shortwave swinging out of the southern Rockies and lead to a Marginal Risk of Excessive Rainfall. As the day goes on convection is expected to become predominately elevated behind the arctic cold front as PWATs gradually fall below 1 inch. Still, MUCAPE of 500-1000 J/kg and weak 850 hpa flow could support localized 2-3 inch rainfall amounts as the scattered showers/storms gradually progress east-southeast through late this evening. ....Southeast Georgia and the Florida Peninsula... There is a non-zero chance for excessive rainfall associated with northeast to southwest oriented bands of convection well separated from the center of Nicole, but still associated with the large synoptic circulation. Subtle areas of low level convergence amid a moist and unstable airmass may lead to localized areas of intense rainfall rates. Central and southern Florida will have the better instability (500-1500 J/kg SB CAPE), while southeast Georgia will have a better upper-level shear profile. Urbanized areas will be most at risk to any slow-moving or training convection that develops. Snell Day 2 Valid 12Z Sat Nov 12 2022 - 12Z Sun Nov 13 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN NEW ENGLAND... As the remnant low of Nicole crosses over northern New England on Saturday, moderate to heavy rainfall associated with frontogenetical forcing around the extratropical system will continue to produce localized 1 inch totals through 21z. PWATs are forecast to remain quite anomalous for this time of year over the region and into the 1.5-1.75" range. Precluding moisture within a warm air advection regime is likely to saturate soils ahead of this final surge of rainfall, leading to localized flash flooding potential. The progressiveness of the system and lack of instability supports only a Marginal Risk area, which is unchanged from the previous forecast. Snell Day 3 Valid 12Z Sun Nov 13 2022 - 12Z Mon Nov 14 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4Y79s5Zc1jiaPpYR-FncmlRWJ1NyDCnN-_FZ57LWoZso= shpVRMg5jsE93gUfVstHuJCD0cymHZkES9cv8Qi22mWWtjs$=20=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Y79s5Zc1jiaPpYR-FncmlRWJ1NyDCnN-_FZ57LWoZso= shpVRMg5jsE93gUfVstHuJCD0cymHZkES9cv8Qi2vrH6zsk$=20=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Y79s5Zc1jiaPpYR-FncmlRWJ1NyDCnN-_FZ57LWoZso= shpVRMg5jsE93gUfVstHuJCD0cymHZkES9cv8Qi2z30s00I$=20=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .